Effects of metabolic alterations on dopamine release in an in vitro model of neostriatal ischaemia

Physiological aging at striatal synapses

Mike Levine's body of work guides thinking on how the basal ganglia process information to create coordinated movements and skill learning throughout the life span and in disease. This special issue is a nod to Mike's career and a well-deserved gesture by the neuroscience community thanking him for the impact he has made on many people's careers and the field of basal ganglia physiology. This paper reviews how aging impacts basal ganglia processing with a focus on single cell and synaptic physiology. This review begins with the work Mike did with his collaborators Nat Buchwald, Chester Hull and Jay Schneider. These early studies paved the way for subsequent studies on changes in synaptic processing that occur with aging in the basal ganglia. The primary focus of this review is aging at corticostriatal synapses. Corticostriatal synapses show reduced expression of both short-term and long-term synaptic potentiation. The roles of age-related changes in calcium homeostasis, vesicle cycling, dopamine modulation, and NMDA receptor function in aging's effect on synaptic plasticity are discussed. The article ends with a review of mitochondrial aging theory as it applies to age-induced changes in corticostriatal synaptic function.

Protective role of Kv7 channels in oxygen and glucose deprivation-induced damage in rat caudate brain slices

Ischemic stroke can cause striatal dopamine efflux that contributes to cell death. Since Kv7 potassium channels regulate dopamine release, we investigated the effects of their pharmacological modulation on dopamine efflux, measured by fast cyclic voltammetry (FCV), and neurotoxicity, in Wistar rat caudate brain slices undergoing oxygen and glucose deprivation (OGD). The Kv7 activators retigabine and ICA27243 delayed the onset, and decreased the peak level of dopamine efflux induced by OGD; and also decreased OGD-induced damage measured by 2,3,5-triphenyltetrazolium chloride (TTC) staining. Retigabine also reduced OGD-induced necrotic cell death evaluated by lactate dehydrogenase activity assay. The Kv7 blocker linopirdine increased OGD-evoked dopamine efflux and OGD-induced damage, and attenuated the effects of retigabine. Quantitative-PCR experiments showed that OGD caused an ~6-fold decrease in Kv7.2 transcript, while levels of mRNAs encoding for other Kv7 subunits were unaffected; western blot experiments showed a parallel reduction in Kv7.2 protein levels. Retigabine also decreased the peak level of dopamine efflux induced by L-glutamate, and attenuated the loss of TTC staining induced by the excitotoxin. These results suggest a role for Kv7.2 in modulating ischemia-evoked caudate damage.

Dopamine release regulation by astrocytes during cerebral ischemia

Brain ischemia triggers excessive release of neurotransmitters that mediate neuronal damage following ischemic injury. The striatum is one of the areas most sensitive to ischemia. Release of dopamine (DA) from ischemic neurons is neurotoxic and directly contributes to the cell death in affected areas. Astrocytes are known to be critically involved in the physiopathology of cerebrovascular disease. However, their response to ischemia and their role in neuroprotection in striatum are not completely understood. In this study, we used an in vitro model to evaluate the mechanisms of ischemia-induced DA release, and to study whether astrocytes modulate the release of DA in response to short-term ischemic conditions. Using slices of adult mouse brain exposed to oxygen and glucose deprivation (OGD), we measured the OGD-evoked DA efflux using fast cyclic voltammetry and also assessed metabolic impairment by 2,3,5-triphenyltetrazolium chloride (TTC) and tissue viability by propidium iodide (PI) staining. Our data indicate that ischemia induces massive release of DA by dual mechanisms: one which operates via vesicular exocytosis and is action potential dependent and another involving reverse transport by the dopamine transporter (DAT). Simultaneous blockade of astrocyte glutamate transporters and DAT prevented the massive release of dopamine and reduced the brain tissue damage. The present results provide the first experimental evidence that astrocytes function as a key cellular element of ischemia-induced DA release in striatum, constituting a novel and promising therapeutic target in ischemia.

Inhibition of pre-ischeamic conditioning in the mouse caudate brain slice by NMDA- or adenosine A1 receptor antagonists

Evidence suggests that pre-ischeamic conditioning (PIC) offers protection against a subsequent ischeamic event. Although some brain areas such as the hippocampus have received much attention, the receptor mechanisms of PIC in other brain regions are unknown. We have previously shown that 10 min oxygen and glucose deprivation (OGD) evokes tolerance to a second OGD event in the caudate. Here we further examine the effect of length of conditioning event on the second OGD event. Caudate mouse brain slices were superfused with artificial cerebro-spinal fluid (aCSF) bubbled with 95%O₂/5%CO₂. OGD was achieved by reducing the aCSF glucose concentration and by bubbling with 95%N₂/5%CO₂. After approximately 5 min OGD a large dopamine efflux was observed, presumably caused by anoxic depolarisation. On applying a second OGD event, 60 min later, dopamine efflux was delayed and reduced. We first examined the effect of varying the length of the conditioning event from 5 to 40 min and found tolerance to PIC increased with increasing duration of conditioning. We then examined the receptor mechanism(s) underlying PIC. We found that pre-incubation with either MK-801 or 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) reduced tolerance to the second OGD event. These data suggest that either N-methyl-d-aspartate (NMDA) or adenosine A₁ receptor activation evokes PIC in the mouse caudate.

Effect of pre-ischaemic conditioning on hypoxic depolarization of dopamine efflux in the rat caudate brain slice measured in real-time with fast cyclic voltammetry

Fast cyclic voltammetry can be used to measure dopamine release after oxygen and glucose deprivation (OGD) induced anoxic depolarization in vitro. Here we measure dopamine efflux with 1s time resolution, which is appropriate to measure OGD-evoked dopamine efflux accurately. In the present study, we examined whether OGD-evoked dopamine efflux could be used to show pre-ischaemic conditioning in the rat caudate brain slice. Caudate slices were exposed to 0, 2, or 10 min OGD pre-ischaemic conditioning, then 60 min later exposed to a second OGD event of 15 min duration. We measured the OGD-evoked dopamine efflux using fast cyclic voltammetry and in some experiments caudate dopamine and DOPAC tissue levels were measured using HPLC and 20 μm cryostat sections were Nissl stained to indicate neuronal loss. We found that 10 but not 2 min OGD pre-ischaemic conditioning resulted in a longer time to onset of OGD-evoked dopamine efflux on the main OGD event (475 ± 31 and 287 ± 30 s for 10 Vs 0 min pre-ischaemic conditioning respectively). Further, 10 min OGD pre-ischaemic conditioning resulted in less dopamine efflux on the second OGD event (4.23 ± 1.12 and 8.14 ± 0.82 μM for 10 Vs 0 min pre-ischaemic conditioning respectively), despite these slices having similar tissue dopamine content and DOPAC/DA ratio, and the rate of dopamine release was slower in the main OGD event (21 ± 5 and 74 ± 8 nM/s for 10 Vs 0 min pre-ischaemic conditioning respectively). These data suggest that 10 min OGD pre-ischaemic conditioning can evoke tolerance to a second OGD event and that voltammetric recording of OGD-evoked dopamine efflux is a useful model of pre-ischaemic conditioning in neuronal tissue.

Acute and long-term response of dopamine nigrostriatal synapses to a single, low-dose episode of 3-nitropropionic acid-mediated chemical hypoxia

The goal of the present investigation was to determine the persistence of striatal (DA) dopaminergic dysfunction after a mild chemically induced hypoxic event in Fisher 344 rats. To this end, we gave a single injection of the mitochondrial complex II inhibitor 3-nitropropionic acid (3-NP; 16.5 mg/kg, i.p.) to 2-month old male F344 rats and measured various indices of striatal DA functioning and lipid peroxidation over a 3-month span. Separate groups of rats were used to measure rod walking, evoked DA release, DA content, malondialdehyde (MDA) accumulation, DA receptor binding, and tyrosine hydroxylase (TH) activity. The results showed that 3-NP exposure reduced most measures of DA functioning including motoric ability, DA release, and D(2) receptor densities for 1 to 3 months postdrug administration. Interestingly, DA content was reduced 1 week after 3-NP exposure, but rose to 147% of control values 1 month after 3-NP treatment. MDA accumulation, a measure of lipid peroxidation activity, was increased 24 h and 1 month after 3-NP treatment. 3-NP did not affect TH activity, suggesting that alterations in DA functioning were not the result of nigrostriatal terminal loss. These data demonstrate that a brief mild hypoxic episode caused by 3-NP exposure has long-term detrimental effects on the functioning of the nigrostriatal DA system.

Decreased striatal dopamine release underlies increased expression of long-term synaptic potentiation at corticostriatal synapses 24 h after 3-nitropropionic-acid-induced chemical hypoxia

The striatum is particularly sensitive to the irreversible inhibitor of succinate dehydrogenase 3-nitropropionic acid (3-NP). In the present study, we examined early changes in behavior and dopamine and glutamate synaptic physiology created by a single systemic injection of 3-NP in Fischer 344 rats. Hindlimb dystonia was seen 2 h after 3-NP injections, and rats performed poorly on balance beam and rotarod motor tests 24 h later. Systemic 3-NP increased NMDA receptor-dependent long-term potentiation (LTP) at corticostriatal synapses over the same time period. The 3-NP-induced corticostriatal LTP was not attributable to increased NMDA receptor number or function, because 3-NP did not change MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine] binding or NMDA/AMPA receptor current ratios. The LTP seen 24 h after 3-NP was D(1) receptor dependent and reversed by exogenous addition of dopamine or a D(2) receptor agonist to brain slices. HPLC and fast-scan cyclic voltammetry revealed a decrease in dopamine content and release in rats injected 24 h earlier with 3-NP, and much like the enhanced LTP, dopamine changes were reversed by 48 h. Tyrosine hydroxylase expression was not changed, and there was no evidence of striatal cell loss at 24-48 h after 3-NP exposure. Sprague Dawley rats showed similar physiological responses to systemic 3-NP, albeit with reduced sensitivity. Thus, 3-NP causes significant changes in motor behavior marked by parallel changes in striatal dopamine release and corticostriatal synaptic plasticity.

Adenosine and ATP-sensitive potassium channels modulate dopamine release in the anoxic turtle (Trachemys scripta) striatum

Excessive dopamine (DA) is known to cause hypoxic/ischemic damage to mammalian brain. The freshwater turtle Trachemys scripta, however, maintains basal striatal DA levels in anoxia. We investigated DA balance during early anoxia when energy status in the turtle brain is compromised. The roles of ATP-sensitive potassium (K(ATP)) channels and adenosine (AD) receptors were investigated as these factors affect DA balance in mammalian neurons. Striatal extracellular DA was determined by microdialysis with HPLC in the presence or absence of the specific DA transport blocker GBR-12909, the K(ATP) blocker 2,3-butanedione monoxime, or the nonspecific AD receptor blocker theophylline. We found that in contrast to long-term anoxia, blocking DA reuptake did not significantly increase extracellular levels in 1-h anoxic turtles. Low DA levels in early anoxia were maintained instead by activation of K(ATP) channels and AD receptors. Blocking K(ATP) resulted in a 227% increase in extracellular DA in 1-h anoxic turtles but had no effect after 4 h of anoxia. Similarly, blocking AD receptors increased DA during the first hour of anoxia but did not change DA levels at 4-h anoxia. Support for the role of K(ATP) channels in DA balance comes from normoxic animals treated with K(ATP) opener; infusing diazoxide but not adenosine into the normoxic turtle striatum resulted in an immediate DA decrease to 14% of basal values within 1.5 h. Alternative strategies to maintain low extracellular levels may prevent catastrophic DA increases when intracellular energy is compromised while permitting the turtle to maintain a functional neuronal network during long-term anoxia.

Effects of creatine treatment on the survival of dopaminergic neurons in cultured fetal ventral mesencephalic tissue

Parkinson's disease is a disabling neurodegenerative disorder of unknown etiology characterized by a predominant and progressive loss of dopaminergic neurons in the substantia nigra. Recent findings suggest that impaired energy metabolism plays an important role in the pathogenesis of this disorder. The endogenously occurring guanidino compound creatine is a substrate for mitochondrial and cytosolic creatine kinases. Creatine supplementation improves the function of the creatine kinase/phosphocreatine system by increasing cellular creatine and phosphocreatine levels and the rate of ATP resynthesis. In addition, mitochondrial creatine kinase together with high cytoplasmic creatine levels inhibit mitochondrial permeability transition, a major step in early apoptosis. In the present study, we analyzed the effects of externally added creatine on the survival and morphology of dopaminergic neurons and also addressed its neuroprotective properties in primary cultures of E14 rat ventral mesencephalon. Chronic administration of creatine [5 mM] for 7 days significantly increased survival (by 1.32-fold) and soma size (by 1.12-fold) of dopaminergic neurons, while having no effect on other investigated morphological parameters. Most importantly, concurrent creatine exerted significant neuroprotection for dopaminergic neurons against neurotoxic insults induced by serum and glucose deprivation (P < 0.01), 1-methyl-4-phenyl pyridinium ion (MPP+) [15 microM] and 6-hydroxydopamine (6-OHDA) [90 microM] exposure (P < 0.01). In addition, creatine treatment significantly protected dopaminergic cells facing MPP+-induced deterioration of neuronal morphology including overall process length/neuron (by 60%), number of branching points/neuron (by 80%) and area of influence per individual neuron (by 60%). Less pronounced effects on overall process length/neuron and number of branching points/neuron were also found after 6-OHDA exposure (P < 0.05) and serum/glucose deprivation (P < 0.05). In conclusion, our findings identify creatine as a rather potent natural survival- and neuroprotective factor for developing nigral dopaminergic neurons, which is of relevance for therapeutic approaches in Parkinson's disease and for the improvement of cell replacement strategies.

Type I glutaric aciduria, part 2: a model of acute striatal necrosis

Type I glutaric aciduria (GA1) is an inborn error of organic acid metabolism that is associated with acute neurological crises, typically precipitated by an infectious illness. The neurological crisis coincides with swelling, metabolic depression, and necrosis of basal ganglia gray matter, especially the putamina and can be visualized as focal, stroke-like, signal hyperintensity on MRI. Here we focus on the stroke-like nature of striatal necrosis and its similarity to brain injury that occurs in infants after hypoxia-ischemia or systemic intoxication with 3-nitropropionic acid (NPA). These conditions share several features including abrupt onset, preferential effect in the striatum and age-specific susceptibility. The pathophysiology of the conditions is reviewed and a model proposed herein. We encourage investigators to test this model in an appropriate experimental system.

An assessment of the cerebroprotective potential of volatile anaesthetics using two independent methods in an in vitro model of cerebral ischaemia

Previous studies using a rat brain slice model of cerebral 'ischaemia' (hypoxia and hypoglycaemia) have suggested that volatile anaesthetics may have cerebroprotective potential. In this study, we tested the cerebroprotective profile of four volatile anaesthetics in this model by two independent means: voltammetric measurement of 'ischaemia'-induced dopamine (DA) release and post-'ischaemic' tissue staining with 2,3,5-triphenyltetrazolium chloride (TTC). 'Ischaemia' caused a characteristic pattern of DA release. Halothane, isoflurane and enflurane did not affect the time from onset of 'ischaemia' to the initiation of DA release. However, all three volatile agents significantly increased (P<0.01, P<0.05, P<0.001, respectively) the time taken for 'ischaemia'-induced DA release to reach maximum and reduced the rate of DA release. Enflurane, unlike halothane or isoflurane, reduced the maximal extracellular DA concentration induced by 'ischaemia' (P<0.01). The effects of sevoflurane were inconsistent. At the higher concentrations used, the volatile anaesthetics frequently changed the character of DA release from monophasic to biphasic, an effect only previously seen in this model with Na(+) channel blockers. 'Ischaemia' also diminished the subsequent level of tissue staining with TTC. When the effects of the volatile agents were analysed by TTC staining, only enflurane showed any cerebroprotective effects and these were limited to the striatum (P<0.01). High concentrations of halothane, isoflurane and enflurane appeared to have some 'toxic' effects, reducing TTC staining in control slices. In summary, we do not find any consistent evidence that volatile anaesthetics are cerebroprotective in this model.

Health implications of creatine: can oral creatine supplementation protect against neurological and atherosclerotic disease?

Major achievements made over the last several years have highlighted the important roles of creatine and the creatine kinase reaction in health and disease. Inborn errors of metabolism have been identified in the three main steps involved in creatine metabolism: arginine:glycine amidinotransferase (AGAT), S-adenosyl-L-methionine:N-guanidinoacetate methyltransferase (GAMT), and the creatine transporter. All these diseases are characterized by a lack of creatine and phosphorylcreatine in the brain, and by (severe) mental retardation. Similarly, knockout mice lacking the brain cytosolic and mitochondrial isoenzymes of creatine kinase displayed a slightly increased creatine concentration, but no phosphorylcreatine in the brain. These mice revealed decreased weight gain and reduced life expectancy, disturbed fat metabolism, behavioral abnormalities and impaired learning capacity. Oral creatine supplementation improved the clinical symptoms in both AGAT and GAMT deficiency, but not in creatine transporter deficiency. In addition, creatine supplementation displayed neuroprotective effects in several animal models of neurological disease, such as Huntington's disease, Parkinson's disease, or amyotrophic lateral sclerosis. All these findings pinpoint to a close correlation between the functional capacity of the creatine kinase/phosphorylcreatine/creatine system and proper brain function. They also offer a starting-point for novel means of delaying neurodegenerative disease, and/or for strengthening memory function and intellectual capabilities.Finally, creatine biosynthesis has been postulated as a major effector of homocysteine concentration in the plasma, which has been identified as an independent graded risk factor for atherosclerotic disease. By decreasing homocysteine production, oral creatine supplementation may, thus, also lower the risk for developing, e.g., coronary heart disease or cerebrovascular disease. Although compelling, these results require further confirmation in clinical studies in humans, together with a thorough evaluation of the safety of oral creatine supplementation.

Methyl-4-phenylpyridinium (MPP(+))-evoked dopamine release from rat striatal slices: possible roles of voltage-dependent calcium channels and reverse dopamine transport

We examined the properties of voltage-dependent Ca(2+) channels (VDCCs) mediating 1-methyl-4-phenylpyridinium (MPP(+))-evoked [3H]DA release from rat striatal slices. In some cases, the Ca(2+)-independent efflux of neurotransmitters is mediated by the high-affinity neurotransmitter-uptake systems. To determine whether such a mechanism might be involved in MPP(+)-evoked [3H]DA release. MPP(+) (1,10 and 100 microM) evoked the release of [3H]DA from rat striatal slices in a concentration-dependent manner. In the absence of Ca(2+), MPP(+) (10 and 100 microM)-evoked [3H]DA release was significantly decreased to approximately 50% of control (a physiological concentration of Ca(2+)). In the presence of Ca(2+), nomifensine (0.1,1 and 10 microM) dose-dependently and significantly inhibited the MPP(+)-evoked release of [3H]DA. Nomifensine (1 and 10 microM) also dose-dependently and significantly inhibited the MPP(+)-evoked release of [3H]DA under Ca(2+)-free conditions. MPP(+)-evoked [3H]DA release was partly inhibited by nicardipine (1 and 10 microM), an L-type Ca(2+) channel blocker. On the other hand, the N-type Ca(2+) channel blocker omega-conotoxin-GVIA (omega-CTx-GVIA) (1 and 3 microM) did not affect this release. omega-agatoxin-IVA (omega-Aga-IVA) at low concentrations (0.1 microM), which are sufficient to block P-type Ca(2+) channels alone, also had no effect. On the other hand, MPP(+)-evoked [3H]DA release was significantly decreased by high concentrations of omega-Aga-IVA (0.3 microM) that would inhibit Q-type Ca(2+) channels. In addition, application of the Q-type Ca(2+) channel blocker omega-conotoxin-MVIIC (omega-CTx-MVIIC) (0.3 and 1 microM) also significantly inhibited MPP(+)-evoked [3H]DA release. These results suggest that MPP(+)-evoked [3H]DA release from rat striatal slices is largely mediated by Q-type Ca(2+) channels, and the Ca(2+)-independent component is mediated by reversal of the DA transport system.

Characterization of vinpocetine effects on DA and DOPAC release in striatal isolated nerve endings

The effect of vinpocetine, a nootropic drug with anti-ischemic potential, on the release of DA and its main metabolite, DOPAC, was investigated in striatum isolated nerve endings under resting and depolarized conditions. Vinpocetine does not modify the baseline release of DA or the exocytotic release of DA evoked by high K(+), but inhibits the release of DA evoked by veratridine reversal of the DA transporter. In addition to these results, which confirm the vinpocetine selective blockade of voltage-sensitive presynaptic Na(+) channels (VSSC) previously reported [Neurochem. Res. 24 (1999) 1585], vinpocetine increases DOPAC release either under resting, veratridine or high K(+) depolarized conditions. This latter effect, which does not involve VSSC, was characterized. The parallel determination of the released and retained catecholamine concentrations revealed that vinpocetine increases DOPAC release at the expense of internal DA in a dose-dependent manner (low microM range). In contrast to vinpocetine, the selective MAO-A inhibitor, clorgyline, increases DA and decreases DOPAC formation. The combined action of vinpocetine and clorgyline does not indicate, however, that the activation of MAO is the mechanism responsible for the increase in DOPAC caused by vinpocetine. Reserpine, although more potent and efficient than vinpocetine, qualitatively exerts the same pattern of changes on DA and DOPAC concentrations. It is concluded that, in addition to the inhibition of presynaptic VSSC permeability, which selectively inhibits the transporter-mediated release of all neurotransmitters, vinpocetine increases DOPAC by impairing the vesicular storage of DA. Our results indicate that the cytoplasm extravesicular DA is metabolized by MAO to DOPAC. Most of the DOPAC formed is exported to the extracellular medium.

Effects of sevoflurane on dopamine, glutamate and aspartate release in an in vitro model of cerebral ischaemia

Release of excitatory amino acids and dopamine plays a central role in neuronal damage after cerebral ischaemia. In the present study, we used an in vitro model of ischaemia to investigate the effects of sevoflurane on dopamine, glutamate and aspartate efflux from rat corticostriatal slices. Slices were superfused with artificial cerebrospinal fluid at 34 degrees C and episodes of 'ischaemia' were mimicked by removal of oxygen and reduction in glucose concentration from 4 to 2 mmol litre(-1) for < or = 30 min. Dopamine efflux was monitored in situ by voltammetry while glutamate and aspartate concentrations in samples of the superfusate were measured by HPLC with fluorescence detection. Neurotransmitter outflow from slices was measured in the absence or presence of sevoflurane (4%). After induction of ischaemia in control slices, there was a mean (SEM) delay of 166 (7) s (n = 5) before sudden efflux of dopamine which reached a maximum extracellular concentration of 77.0 (15.2) micromol litre(-1). Sevoflurane (4%) reduced the rate of dopamine efflux during ischaemia (6.90 (1.5) and 4.73 (1.76) micromol litre(-1) s(-1) in controls and sevoflurane-treated slices, respectively; P<0.05), without affecting its onset or magnitude. Excitatory amino acid efflux was much slower. lschaemia-induced glutamate efflux had not reached maximum after 30 min of ischaemia. Basal (pre-ischaemic) glutamate and aspartate efflux per slice was 94.8 (24.8) and 69.3 (31.5) nmol litre(-1) superfusate (n = 4) and was not significantly reduced by 4% sevoflurane. lschaemia greatly increased glutamate and aspartate efflux (to a maximum of 919 (244)% and 974 (489)% of control, respectively). However, ischaemia-induced efflux of both glutamate and aspartate was significantly reduced by 4% sevoflurane (P < 0.001 for glutamate, P < 0.01 for aspartate). In summary, sevoflurane may owe part of its reported neuroprotective effect to a reduction of ischaemia-induced efflux of excitatory amino acids and, to a lesser extent, dopamine.

The reverse transport of DA, what physiological significance?

It is well established that midbrain dopamine neurons innervating the striatum, release their neurotransmitter through an exocytotic process triggered by the neural firing and involving a transient calcium entry in the terminals. Long ago, it had been proposed, however, that another mechanism of release could co-exist with classical exocytosis, involving the reverse-transport of the cytosolic amine by the carrier, ordinarily responsible for uptake function. This atypical mode of release could be evoked directly at the preterminal level by multiple environmental endogenous factors involving transient alterations of the sodium gradient. It cannot be excluded that this mode of release participates in the firing-induced release. In contrast with the classical exocytosis of a preformed DA pool, the reverse-transport of DA requires simultaneous alterations of intraterminal amine metabolism including synthesis and displacement from storage compartment. The concept of a reverse-transport of dopamine is coming from the observations that releasing substances, such as amphetamine-related molecules, actually induce this type of transport. A large set of arguments advocates that reverse-transport plays a role in the maintenance of basal extracellular DA concentration in striatum. It was also often evoked in physiopathological situations including ischemia, neurodegenerative processes, etc. The most recent studies suggest that this release could occur mainly outside the synapses, and thus could constitute a major feature in the paracrine transmission, sometimes evoked for DA.